Thumper The T-Shirt Launcher

[joe] and [ryan] built Thumper for their high school FIRST robotics team. The cannon itself is a solenoid-fired compressed air launcher that gets its juice from three large PVC tanks stored in the box below the turret, and the cannon is able to be fired nine times between visits to the air compressor. It was intentionally designed to resemble an M2 Browning 50 Caliber heavy machine gun, with the two vertical handles and boxy body. They finished construction in about a week with a budget of only $300. When they saw that a lot of their friends had also built cannons, they scrounged for parts from their garages to re-use to build the mobile platform simply for one-upmanship sake. The motor and drive-train propelling this behemoth came out of a 1980s-era mobile X-Ray machine that had been discarded by a local hospital. The rear wheels were specially modified to fit the drivetrain, and the front end is a chopped, hacked, and welded axle and steering mechanism from an old lawn tractor. Sections of unistrut form the rest of the frame.

[joe] and [ryan] were even asked to bring Thumper to their high school prom as a unique way to hand out T-Shirts for the evening. Unfortunately, there’s no website for this build.

See video of Thumper in action with a Nerf Football after the break. Hack A Day even got to take it for a spin around the Power Wheels Racing Series track at Maker Faire KC!

Why is PVC unsafe for storing air when it is used all the time for safely transporting water and other liquids?

Water, like most liquids, is not compressible, therefore it cannot store energy. When a hydrostatic failure occurs, water is projected, but the shrapnel is not projected very far.

On the other hand, air and other gases are compressible. This can result in large amounts of stored energy. System failure could lead to a disastrous situation when this energy is released, sending shrapnel outward. Severe injury and damage can result.

PVC is susceptible to breaking – through physical damage, crushing, sudden impacts, etc. Small scratches on the outside of the pipe can turn into stress points that when combined with air pressure behind them, can lead to sudden and rapid failure – and injury.

Major corporations use PVC for compressed air lines all the time. PVC can be used for compressed air distribution in low pressure systems.

In higher pressure systems, it’s still permissible. even under California building code (largely regarded as one of the stricter states when speaking of building/zoning codes)
Provided that the PVC is contained in (rigid conduit, or underground, or concrete etc..)

Everything is application based, and everything can be legitimate, in the right system with the appropriate design and precautions.

Saying “PVC isn’t safe for compressed air systems because it could rupture” is like saying that “seaside beaches aren’t safe for swimming because you could get eaten by a shark.” Both are realistic outcomes, and VERY difficult to refute. however…

You know, airplanes are really too expensive. Let’s skimp on the expensive materials because we can probably get away with it for a long time – maybe forever. Maybe we can use PVC for the cockpit window (which need to be bird strike rated)? Or to carry fuel in? Or even better – for hydraulic applications!

Also, crumple zones add a lot of cost to a car. We should not put those in to save money. Better still, because we saved money there and saved cost on weights, we can sell the less safe car as more fuel efficient! It’s win-win!

The second one would be more laughable if it were not in fact happening to automobiles in far more cost conscious countries such as India and China.

I am all for saving money where possible and for reusing materials as well as trying to balance the expected use of a project vs the cost. But there are some things you just don’t skimp on unless you truly have no other choice (life or death situations).

Even if the project has a strict budget, you still should not skimp on safety. But to each their own.

high school science project vs transportation industry. wonderful comparison.
ive got an idea, lets not teach our kids about technology because in order to do that, apparently we need the same multimillion dollar materials NASA uses. its not acceptible to just use a pencil, like the russians. better yet, lets stop teaching science in our schools all together. go back to the faerytales of religion.

The problem with PVC is that even pressure rated parts, are rated for water pressure.

Hydrostatic testing is used to find the burst pressure of PVC pipe…. when filled with water, PVC will split open, and crack…. when filled with air PVC will explode violently and send shards of radiolucent plastic in all directions with lethal force.

With that said, i used PVC for air all the time…simply keep it under 100 psi, avoid C02, avoid cold temps, and well as very hot temp, never drop it, and you will be fine.

I suggest everyone checks out a forum called “spudfiles” to see a few hundred air powered home made cannons.

I think the comparison was valid, it was about the tradeoff between cost and safety. No one is advocating putting kids in a bubble because science is too dangerous, but this is a case where there is a serious danger. You wouldn’t want kids working in chemistry labs without safety goggles would you? The fact is, the manufacturer explicitly states that PVC should NOT be used for compressed air, because failure of PVC is catastrophic. Instead, they should have used ABS, which fails under pressure much more gracefully (and safely). While there will always be inherent dangers in science, they should be avoided if the risks posed are very serious and/or a safer solution is readily available.

PVC as a compressed air storage tank scares the hell out of me, even if some of the other applications I’ve seen aren’t so worrisome. It’s not terribly expensive to acquire a proper steel air tank, and is much less terrifying.

For what it’s worth, all the pressurized vessels are inside pretty sturdy wooden boxes with pressure relief. They aren’t putting much pressure into the lower tanks, as they’re all three much larger in volume than the charge tank that feeds the launcher.

A common hackerspace and Burning Man mantra is “Safety Third!” and I kind of like that approach. Without a few risks in life, it’s hard to get anywhere fun.

I am all about taking some risks. But take measured ones and don’t tempt KNOWN risks that have, time and time again, proven to be very bad risks.

We went to the moon (several times) and returned safely to earth. We took huge risks in doing so and given the enormity of the task, fairly few lives have been list in doing so. Obviously the budget and scope aren’t the same here but I am trying to make a point about the approach to design and implementation – I don’t care if it is the car industry or a high school project. The same concepts apply.

When you have a known risk, one that has been discussed at length, has a 20+ year track record of being a problem and can even be easily tested yourself under controlled settings AND the very manufacturers warn against said using their very own products in said applications, even if it is “cheap” – you are an idiot for ignoring all of those factors and proceeding ahead anyway.

I have seen burning man fire throwers. They may not all be highly engineered, super safe devices. But at least the makes of said fire throwers (tend to) have the common sense to use a properly rated gas tank.

If cost is the primary concern, it need not be. Go buy a cheap, steel, air compressor tank at Wal-mart if you absolutely must save money. Harbor Freight has ones with the volume needed for under $75. You probably will wind up spending the same amount on large diameter PVC pipe (that stuff is hardly cheap in large sizes).

I am glad there is some pressure relief to prevent over pressure bursting although that still doesn’t address the catastrophic failure at working pressure problem. But at least the PVC is inside a wooden crate – that helps some too in the event of a catastrophic rupture.

My point was that the transportation industry does it right because it has to be done that way or products would be unsafe. Would you drive a car that was made by an average high school shop class student?

“ive (punctuation) got an idea, lets not teach our kids about technology because in order to do that, apparently we need the same multimillion dollar materials NASA uses.”

I didn’t say that. Just buy an air tank from Wal-Mart for the same price or possibly even less than the price of the pipe.

“its not acceptible (spelling) to just use a pencil, like the russians (capitalization).”

That’s actually a myth / urban legend that the Russians just used a pencil and the US used a million dollar, highly engineered pen.

“better yet, lets stop teaching science in our schools all together. go back to the faerytales (spelling) of religion.”

Who said anything about that? What does religion have to do with science? I love science. I think it should be taught more. Just teach it responsibly and avoid known risks, like any good science teacher generally tries to do.

When I was in high school my friend and I build a water balloon launcher and used PVC for air storage. I new the danger and asked about the pressure rating of the pipe, it was 300 psi. The problem was the fittings. The end cap blew out while we where releasing pressure from the tank. Luckily the end pointing away from us is the one that blew out.

I guess spelling is important to many science and engineering folks because we think details and truths are important. What’s the point of having different words, spellings, grammar and syntax if we’re not going to use them to communicate clearly. It’s just as easy to do write properly as it is to do it incorrectly.

Stupidity and ignorance should not be mistaken for bravery and attempting to break new ground. There’s nothing new here. Safety and science go hand-in-hand. Safety knowledge and practices are based on science anyway, why are they incompatible? Better to be around or able to do your next project than to be maimed / killed doing this one because you were to dumb or lazy to take heed that what you were doing was well known to be high risk with potentially catastrophic results.

Let’s put it another, more familiar way. You’re building an AC mains powered tube amp, and at the time (go with me on this) earth wires are difficult to get, so you decide not to bother with one. If you build it right, then you probably wouldn’t have any problems, but if something goes wrong, you’re quite easily toast.

Safety third may very well be a Burning Man mantra if you ignore what those who put on the event publish on the web for all interested, to read. Most likely not the mantra for any hacker space that desires to stay in existence. Stupid to endanger yourself needlessly, but criminal to endanger others. In regards to one’s own safety, serious permanent injury really does put a damper on getting anywhere fun in life.

DIdn’t bother to read all the comments, but a lot of people are saying to go buy a tank from walmart or some other store for the compressed air.

I would suggest going to the local junkyard and asking for a air tank off a semi. They might have a bunch laying around that cant be put on a another semi due to DOT regulations but still hold air and are still safe for use. might be able to get one cheap or for free if your lucky.

Just make sure to check for excessive rust or hairline cracks. another good idea to check its integrity is to fill it with a small amount of air say 50lbs (once it is determined safe for a test) and submerging it in water. if any bubbles show there is some sort of hole and it is unsafe for use.

Just as an FYI, in most states it is illegal to use PVC for compressed air in commercial applications and possibly home applications as well. Check your local laws.

Perfectly safe? no, but all of you wining about pvc bursting so easily have obviously not worked with it much. These pipes are tough, the burst pressures are 3-5 times higher than the rated pressures. I’ve been building cannons for years, and I’ve never seen or even heard of one exploding.

(actually, I have heard of cannons exploding when people don’t let the glue cure, but those people are idiots)

The issue isn’t that they can’t take the static pressure. Obviously they can. The issue is when PVC experiences a sudden impact or are damaged (even scratches allow failures to propagate). This may happen accidentally and without warning.

PVC fails much easier than you may think – a few pounds of weight moving towards a sealed pipe with a modest amount of air behind it is all it takes. Especially when they are cold – PVC gets even more brittle.

Moving to schedule 80 vs schedule 40 helps some but the larger diameter stuff (which has more ability to store potential energy) and compressed air is just asking for trouble.

I don’t like to work with things that literally act like small bombs complete with shrapnel if you accidentally drop them or something hits them.

So – that’s the concern here. That’s all. Use a container to hold compressed air that is properly pressure rated and that has the structural integrity to hold up to reasonable and possibly unforeseen stresses / impacts / etc.

There is no problem with air used in PVC. A PSI of water is the same as a PSI of air. I have worked in many machine shops that used PVC for air. I have never see one failure. I would recommend a cover over the PVC to catch anything just in case. On the other hand I have seem copper pipe being used for compressed air and have seen failures.

That’s why the racing industry has blower straps on Superchargers. Also scatter shield blankets over transmissions.

If your going to push the limits (like racing), then you need proper safety gear.

You are correct in that it exerts the same force upon the walls of the container.

However the point I have lamented at length is that 1) air compresses, water doesn’t (well, it does but only under fairly extreme pressures) and that means that when the compressed air is released it is more like a spring being unsprung – that process is far more hazardous than water being released because water doesn’t appreciably compress to a smaller volume like air does at PVC pressure ranges and 2) that PVC is brittle and fails in catastrophic ways upon impacts vs other more suitable containers such as steel. If you hit a pressurized steel tank with a sledgehammer – it will likely hold up. Do the same with an equally pressurized and sized PVC tank and the results may not be pretty.

Can you get away with using PVC as an air tank? Sure. Should you? I personally and the manufacturers of PVC fittings don’t think you should. You are presumably an adult – do with that what you will.

@HaD – Where’s a link with detailed info on a build process? The makerfaire link has nothing to do with the build.

There’s a difference between a transmission blanket, for exploding clutches and parts, and duct-tape or glued-denim (??) on a PVC chamber, and part of that is the fact your face or arm isn’t an inch or two away from the potential explosion.

Also, go ahead and put a single layer of duct-tape on an M80 and light it in your hand and let me know how that goes. Sure, you may not have a million pieces of shrapnel.. only one exit point with all of the power being released instead.

The steel vessels on Amazon or at other places would require drilling and re-welding larger bungs, as most of them come with 1/4″ NPT and wouldn’t flow as much as they needed. At which point you damage the inner protective lining, but yeah it would be overall a little safer.

Hello all,
My name is Joseph (Joe) I am in fact the primary builder in charge of this project. First off, I would like to make the statement that all of you “whiny NO BALLS key slappers”. No offense to “NON WHINY KEY SLAPPERS”, especially you supportive ones, you are awesome, ballsy, and what helps makes technology continue to progress. All of you (whiny key slappers) are the reason I do not currently have a facebook, web site, or other way you can find and impersonally critique me and or my fellow makers.
Secondly let me clarify, the cart was made first of an estimated 90% recycled materials in a weeks time. Originally deigned and used for the 2011 KC regionals FRC to haul the teams robot from the pit to the arena. Now for the launcher and turret bit, it consists of an estimated 80% recycled materials. It was made after the competition when the team teachers came to me asking that I be the foreman and prime builder of a t-shirt launcher, giving me no guidelines except for the completeion date. I cannot express to all of you enough how many precautions we took in the process of this build. I agree that we should have used a certified tank but is was an executive decision by myself that it would be best for the time and materials that we had readily available to us. Also an ideal learning experience for basic tools and procedures for the students involved. You see when I personally take on a project that needs a specific deadline, I need it done on time and within the budget range of the person or organization funding the project.
Thirdly the technical regulations for the device are as follows. We charge each of the three storage tanks to 125 psi and always carefully transport them individually to and from the compressor. We do not intentionally pressurize the firing chamber over 70psi which will send a t-shirt 90 yards at the correct angle. We usually will launch t-shirts on average 30-40 psi which will lob them approx 40-60 ft to crowd members. The device is key operated with Ryan and myself being the only 2 existing key holders. We designed the crate around it from 3/4 in plywood securely fabricated to house the launcher and allow for possible interior airburst pressure deflection.
Fourthly there was a slight miscommunication between the editors of this post and ourselves. There were no other “friends” who had built any, that was probably related to the reason that I had been selected for the build. That was because of the teams failed attempt at one in the previous year. The second miscommunication was the for statement “they scrounged for parts from their garages to re-use to build the mobile platform simply for one-upmanship sake. It was not for one-upmanship sake or just for the turret, the entire thing you see in the picture besides the launcher, air fittings, lights, rear wheels, cart battery, paint and select hardware. were recycled from a totaling three garages and one shed.
To all of you who are going to continue to criticize me or any other supportive person involved in this particular or future post. I cordially invite you to lick my Sweaty, Hard Working, Wrench Turning, key typing pits also I failed English and many other classes many times but I think I have done quite well on this particular “essay” so I’m going to pat my self on the back. :) sincerely -Joe- (the builder)
p.s. Without risk how can we as a people make significant progess in anything we do? Joseph A. i.e. me.